This project involves various aspects of the cat's visual system with the main emphasis on the functional organization of center-surround type retinal ganglion cell receptive fields. Since knowledge of the true spatial distribution of light within the retinal image is essential for conclusions based on ganglion cell experiments which employ stimuli restricted to various portions of the receptive field, some effort is also devoted to exploring the relevant aspects of the cat's physiological optics. The general health of the retina is another important factor which determines how valid are interpretations of ganglion cell responses. Simultaneous recording of (a) ganglion cell responses and (b) changes in retinal oxygen tension brought about by changes in inhaled gases is therefore also included in our experiments. Some experiments involving cortical neurons and masking techniques are also pursued. The animals are under light general anesthesia (i.v. urethane); action potentials are recorded from single axons in the optic tract and well-focused stimuli are provided by a Maxwellian view stimulator or by a display on an oscilloscope screen.